Integrated circuit devices typically include a single semiconductor chip in which a number of functional elements are defined as is well understood. These functional elements could include a programmable logic array (PLA), an instruction register, memory, bus lines, etc. as would be present in a microprocessor. These functional elements are laid out on the chip in a manner to conserve space and are connected electrically to peripherally placed lands or pads. Electrical connection to the outside world is made to these pads typically through a hermetically-sealed, dual-in-line package (DIP pack) characterized by two parallel rows of pins.
Electrical interface circuits such as input buffers are commonly associated with some of the peripherally placed pads. Often difficult electrical interface circuits such as output drivers are associated with other pads. Also, both an input buffer and an output driver may be associated with a single pad. Input buffers provide an impedance match to that of the driver as well as any voltage or current translation which may be required. Output drivers provide the voltage and current levels necessary to interface to other chips or devices (i.e., LEDs, transistors, etc.). Both buffers and drivers provide protection from voltage spikes due, for example, to static charge build-up--an especially important consideration for MOS logic, from which many integrated circuits are fabricated.
Many situations exist in which the driver and buffer associated with each pad are unnecessary in the final package; for example, they are necessary only for chip testing to interface with existing test systems. The drivers and buffers, of course, require significant amounts of space and consume significant power. Consequently, the inclusion of the buffers and drivers necessitates larger packages to house the elements and to dissipate heat. Particularly in cases where the final packaged device is to be connected to another integrated circuit with similar input/output characteristics with a minimum of capacitance, the buffers and drivers could be eliminated or reduced in size significantly.
But as stated hereinbefore, the buffers and drivers are necessary for testing. Ideally, the buffers and drivers could be included in the test machine probe card. But, this approach appears unlikely because the capacitance of the test probes alone exceeds the drive capability of the unbuffered MOS drivers. The problem thus is to provide a semiconductor chip with adequate drive capability for testing, yet eliminate unnecessary circuitry in the final product.